1. Field of the Invention
The present invention relates in general to continuously variable transmissions, and more particularly to the continually variable transmissions of a friction roller type.
2. Description of the Prior Art
In order to clarify the task of the present invention, a known friction roller type continuously variable transmission will be described with reference to FIGS. 9 and 10 of the accompanying drawings. Such transmission is shown in Japanese Patent First Provisional Publication 1-295070.
For ease of description, the friction roller type continuously variable transmission will be referred to as friction roller type-CVT hereinafter.
FIG. 9 shows a part of the friction roller type-CVT of the Publication, which includes a servo-piston for carrying out the speed change control of the transmission.
In the drawing, denoted by numerals 1 and 2 are input and output cone discs which are arranged on a common rotation axis "O1", and denoted by numerals 20 and 20 are friction rollers (or power rollers) which are each operatively interposed between the input and output cone discs 1 and 2. The friction rollers 20 and 20 are arranged to face each other with the axis "O1" placed therebetween, as shown. That is, each friction roller 20 is put in a toroidal recess defined by both the input and output cone discs 1 and 2, while frictionally contacting with these discs 1 and 2. The friction rollers 20 and 20 are rotatably supported by respective trunnions 86 and 86 (or supporting members) through respective eccentric shafts 82 and 82. The input and output cone discs 1 and 2, the two friction rollers 20 and 20 and the trunnions 86 and 86 constitute a single toroidal power transmission unit.
In a double cavity toroidal type-CVT, two, that is, front and rear toroidal power transmission units are employed, which are coaxially arranged on the axis "O1" with their output cone discs 2 and 2 connected in a back-to-back connecting manner.
As is seen from FIG. 9, the trunnions 86 and 86 have respective upper ends connected through an upper link 80 and respective lower ends connected through a lower link 81. The connection of these parts is so made that each trunnion 86 can pivot between a neutral position as shown in FIG. 9 wherein a rotation axis "O2" of the friction roller 20 intersects the rotation axis "O1" of the input and output cone discs 1 and 2 and an offset position wherein the trunnion 86 is inclined toward a pivot axis "O3" of the friction roller 20 which intersects the rotation axis "O2" at right angles. The trunnion 86 pivots about the pivot axis "O3".
For carrying out the above-mentioned offset movement, each trunnion 86 has a shaft 86d connected to a lower end thereof through a pin 86e, the shaft 86d extending in the direction of the pivot axis "O3". Tightly disposed on the shaft 86d is a servo-piston 108a which is axially movably received in a cylinder body 15.
As is seen from the drawing, the shaft 86d of one (or the left one in the drawing) of the trunnions 86 and 86 has a lower end projected downward beyond a control valve body 18, to which a precess cam 116 is connected. The control valve body 18 has a control valve for generating a hydraulic pressure to operate the servo-piston 108a. A speed change link 21 is incorporated with the precess cam 116, through which an offset degree of the trunnion 86 in the direction of the pivot axis "O3" and a rotation degree of the trunnion 86 about the pivot axis "O3" are fed back to a speed control valve 17 in the control valve body 18.
As shown, the left shaft 86d having the precess cam 116 connected thereto and a hollow boss portion 108b of the servo-piston 108a for receiving the left shaft 86d pass through the cylinder body 15 and the control valve body 18. While, the right shaft 86d having no precess cam and a hollow boss portion 108b of the servo-piston 108a for receiving the right shaft 86d pass through only the cylinder body 15. Each shaft 86d has at a lower end a nut 32 secured thereto for achieving a united connection between the shaft 86d and the associated servo-piston 108a.
Upon receiving a speed change ratio command, the speed control valve 17 in the valve body 18 applies the servo-pistons 108a with a hydraulic pressure corresponding to the content of the command. With this, each servo-piston 108a forces through the corresponding shaft 86d the trunnion 86 to pivot between the above-mentioned neutral position of FIG. 9 and the above-mentioned offset position. Upon this, each friction roller 20 is pivoted about the pivot axis "O3" while bearing a component force applied thereto from the input and output cone discs 1 and 2, so that the rotation speed of the output cone disc 2 is continuously varied relative to that of the input cone disc 1. That is, due to continuously changing contact points of the friction rollers 20 and 20 to the input and output cone discs 1 and 2, the speed change ratio is continuously varied between the input and output cone discs 1 and 2. That is, a continuously variable speed change is carried out.
During the variable speed change, the above-mentioned feedback control is effected, so that with progression of the speed change, the trunnions 86 and 86 are forced to return toward their original positions, and when the existing speed change ratio becomes in agreement with a target value of the command, the trunnions 86 and 86 are returned to the original positions bringing the friction rollers 20 and 20 back to their neutral positions. With this, the speed change ratio can be kept at the target value.
However, due to its inherent construction, the above-mentioned conventional friction roller type-CVT tends to show the following phenomenon under operation thereof.
That is, under operation of the transmission, each friction roller 20 is hardly pressed by and between the input and output cone discs 1 and 2 with a force corresponding to a delivered torque therebetween because the torque delivery has to be made by shearing oil film placed between the friction roller 20 and each of the input and output cone discs 1 and 2. Accordingly, as is seen from FIG. 9, under operation of the transmission, each friction roller 20 is applied with a marked thrust "F" in a direction to be driven out from the input and output cone discs 1 and 2. For suppressing undesired displacement of the trunnions 86 and 86 even when such marked thrust "F" is applied thereto, the upper ends and lower ends of the paired trunnions 86 and 86 are connected through the respective upper and lower links 80 and 81, as is described hereinabove.
However, as is understood from FIG. 10, even when the above-mentioned measure is employed to suppress the displacement of the trunnions 86 and 86, the following phenomenon tends to occur. That is, as is shown by phantom lines in the drawing, under operation of the transmission, the marked thrust "F" forces each trunnion 86 to be resiliently deformed slightly but by a certain degree using the upper and lower links 80 and 81 as fulcrums, so that the shaft 86d and the servo-piston 108a are forced to incline, as shown. This inclination tends to generate a marked friction force between the hollow boss portion 108b of the servo-piston 108a and a wall of a bore of the cylinder body 15 through which the boss portion 108b passes. It has been revealed that such friction becomes severer with increase of the length of the hollow boss portion 108b, like in the case of the longer hollow boss portion 108b to which the precess cam 116 is connected. Of course, in order to carry out smoothed and reliable operation of the friction roller-type CVT, elimination of such friction is needed. If the hollow boss portion 108b is of the longer type to which the precess cam 116 is connected, the feedback control effected by the precess cam 116 becomes poor.